单项选择题

It is said in some parts of the world, goats, rather than cows, serve as a vital ______ of milk.

A. storage
B. reserve
C. resource
D. source
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单项选择题
Joe is not good at sports, but when it （）mathematics, he is the best in the class.

A. comes to
B. comes up to
C. comes on to
D. comes around to
单项选择题
Many people believe we are heading for environmental disaster（）we radically change way we live.

A. but
B. although
C. unless
D. lest
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